This invention relates to an electromagnetic flow meter and, particularly, to an electromagnetic flow meter designed for easy zero-point and span calibration for the flow signal converter and confirmation of preset initial values without suspending the flow rate measurement.
FIG. 1 is a schematic diagram exemplifying the conventional electromagnetic flow meter having a flow sensor and a flow signal converter, and FIG. 2 is a timing chart showing the signal waveforms at various portions of the circuit. In FIG. 1, reference number 2 denotes a flow sensor which consists of excitation windings 4a and 4b, a conduit 6 with insulated interior surface through which liquid under measurement flows, and electrodes 8a and 8b. The excitation windings 4a and 4b produce a magnetic field in a rectangular waveform in the direction perpendicular to the electrode mount position and to the flow of liquid under measurement. In this example, the excitation windings 4a and 4b are connected through semiconductor switch pairs 16, 18 and 20, 22 to a constant current source 12. The constant current source 12 rectifies the power supply current received at the terminal 10 from a power supply source (not shown) and supplies the rectified output to the semiconductor switches 16 and 22. A microprocessor (MPU) 60 produces a reference frequency signal on the basis of the power supply current. The MPU 60 provides a timing signal S1 shown in (a) of FIG. 2 to a switching control circuit 14, which then provides a timing signal S2 ((b) of FIG. 2) to the semiconductor switch pair 16 and 18 and another timing signal S3 ((c) of FIG. 2) to the semiconductor switch pair 20 and 22, so that the switch pairs are turned on and off alternately. This switching operation causes the excitation windings 4a and 4b to flow an excitation current Iex shown in (d) of FIG. 2.
Flow signals generated on the electrodes 8a and 8b are conducted through switches 26 and 28 in the switch circuit 24 for selecting either a measurement mode or an inspection mode and delivered through noise blocking capacitors to the differential amplifying circuit 30. The amplifying circuit 30 includes buffer amplifiers 32 and 34 having high impedance input and a differential amplifier 40. The buffer amplifiers 32 and 34 have variable gain which is controlled through the adjustment of, for example, variable resistors in the gain setting circuits 36 and 38 provided on the feedback path of the amplifiers 32 and 34 in response to the gain switching signal S4 ((i) of FIG. 2). The values of gain are calculated from the inner diameter of the sensor, the flow rate measurement range (span), etc. which the MPU 60 has received from the input setting device 54, and the settings of gain in the circuits 36 and 38 are switched selectively in response to the gain switching signal S4 in accordance with the setting on the input setting device 54.
The differential amplifier 40 produces an output signal having a waveform shown in (e) of FIG. 2, and it is supplied as a flow signal to an inverting amplifier 42 and semiconductor switches 44a and 44b. A positive output of the differential amplifier 40 causes the semiconductor switch 44a to close and semiconductor switch 44b to open, and the flow signal is delivered via the switch 44a to an A/D converter 50. Conversely, a negative output of the differential amplifier 40 causes the semiconductor switch 44a to open and the 44b to close, and the flow signal is reversed for its polarity and then delivered via the switch 44b to the A/D converter 50. The A/D converter 50 produces a signal with a frequency proportional to the flow signal, and it is supplied to the MPU 60. The switches 44a and 44b are controlled by the timing signal S5 ((f) of FIG. 2) from the MPU 60 so that the switch 44a is closed by the timing signal S5 when the flow signal is positive and the switch 44b is closed by the timing signal S5 when the flow signal is negative, for example. Accordingly, the input signal of the A/D converter 50 has a waveform shown in (g) of FIG. 2.
Reference number 48 denotes a calibration signal generator which provides the calibration signal for the A/D converter 50. The signal generator 48 is a d.c. voltage source providing a d.c. voltage signal of a few volts (i.e. a span signal Vc) and another signal of zero volt (i.e. a zero signal Vco), with its positive and negative output terminals connected through switches 46a and 46b, respectively, to the input of the A/D converter 50. The switches 46a and 46b are controlled by the timing signal S6 shown in (j) of FIG. 2 which is synchronized with the timing signal S5, so that the switches 46a and 46b are closed alternately by the timing signal S6 prior to the flow signal sampling operation, i.e. a measurement process cycle, which is performed by alternately closing the switches 44a and 44b as shown in a period of t.sub.7 -t.sub.12 in FIG. 2. Consequently, the zero signal and the span signal of the calibration signal are supplied to the A/D converter 50 through the switch 46a at a period t.sub.2 -t.sub.3 and a period t.sub.5 -t.sub.6, respectively, as shown in a period of t.sub.1 -t.sub.6 in (e) of FIG. 2. The MPU 60 reads the output frequencies of the A/D converter 50 respectively corresponding to the zero and span signals, compares them with the reference values stored in the memory 52 respectively, and, displays the result of comparison. Further, if they do not agree, displays an error message on the display unit 56. Thus, a calibration process cycle is performed in a period of t.sub.1 -t.sub.6 prior to each of the measurement process cycle.
The MPU 60 implements various timing controls and arithmetic operations for the overall circuit. The MPU 60 reads the measurement conditions set on the input setting device 54 including, for example, the inner diameter and span of the sensor and the unit of flow measurement calculates basing on these parameters the value of flow rate from the frequency flow signal provided by the A/D converter 50, and delivers the result of calculation to a D/A converter 58.
The differential amplifier 40 produces the flow rate signal having a waveform shown in a period of t.sub.1 -t.sub.12 in (e) of FIG. 2, and it is shown in more detail for the range T between time points t6 and t12 in FIG. 3. The waveform is made up of four components including a signal voltage Es proportional to the flow velocity of liquid, a magnetically-induced noise voltage MN which arises when the polarity of the excitation current Iex, i.e., the magnetic field, varies on the time axis, an inductive noise voltage IN originating from the commercial power supply, and a d.c. noise voltage Eo created in a loop circuit including the liquid under measurement and the electrodes 8a and 8b, and in the circuit of the converter.
Among the four voltage components, the electromagnetic flow meter needs to extract the signal voltage SV which is proportional to the liquid flow velocity. For this purpose, the switch 44 operates to sample the flow signal during the period ts after the magnetically-induced noise MN caused by the transition of polarity of the excitation current has subsided until the excitation current is cut off. In addition, the flow signal is integrated during the period ts so that the inductive noise IN is averaged out. The MPU 60 implements the following computation in order to remove the d.c. noise Eo. ##EQU1## where Es represents a positive flow signal, -Es' represents a negative flow signal, and Eo represents a d.c. noise voltage.
The flow signal including only the signal voltage component Es (Es') obtained as described above is converted by the D/A converter 58 into a unified electrical signal Io having the amplitude within the range corresponding to the setup span. In this way, the calculated flow signal is delivered every measurement cycle. Thus, the calibration process cycle and the measurement process cycle are performed alternately in a measurement mode. In the calibration process cycle, the calculated value which was obtained in the preceeding measurement process cycle and held in the memory is delivered.
The foregoing electromagnetic flow meter necessitates an inspection operation including the zero-point adjustment which is performed by adjusting the gain of the differential amplifying circuit 30 basing on the measurement of the deviation of characteristics of the circuit 30 as a response to the application of a certain voltage from the standard curve and the calibrating operation for the span. Moreover, this flow meter needs the self diagnostic operation for monitoring as to whether the supply voltage and input signal levels are within the preset normal ranges. The self diagnostic operation takes place during the flow measurement operation, i.e. the measurement mode, whereas the inspection operation needs the switching from the measurmenet mode to an inspection mode for performing the inspection operation by changing over the switches 26 and 28 from the side of the sensor to the side of an inspection signal generator 66 as shown by the dashed line, i.e., the inspection operation requires the power supply to the overall flow meter to be turned off temporarily, which means the suspension of measurement.
In the inspection mode, the power supply to the circuit is activated except for the sensor. The inspection signal generator 66 supplies the inspection voltages (a pregiven voltage of a few millivolts) of opposite polarities from the d.c. power source 64 to the terminals 26b and 28b of the switches 26 and 28 alternately. The inspection voltage Ve has a waveform shown in (h) of FIG. 2, and the Ve is equivalent to the output signals from the electrodes 8a and 8b, respectively. At first, the switches 46a and 46b are closed alternately to deliver the calibration signals to the A/D converter 50 to thereby perform the calibration process cycle. Then, the inspection signal Ve is applied to the A/D converter 50 as shown in a period of t.sub.6 -t.sub.12 of (h) of FIG. 2 and the output thereof is estimated to thereby perform as inspection process cycle. Thus, in the inspection process cycle, the MPU 60 reads the output of the differential amplifier 40 corresponding to the inspection signal Ve through the A/D converter and displays the deviation of the zero-point etc. from the zero point or span point of the flow meter calibration curve on the display unit 56. Thus, in the inspection mode, the calibration process cycle and the inspection process cycle are alternately performed. However, during the operation in the inspection mode, the sensor 2 does not produce the measurement output and so the measurement process cycle is not performed, so that the measurement result is absent during the calibration period.